Chlorinated benzenes are frequently produced by chlorinating benzene or chlorinated benzenes of lower chlorine content in the presence of a catalyst. The formation of the desired chlorinated benzene is accompanied by the formation of various by-products such as more highly chlorinated benzenes. Often isomer(s) of the desired chlorinated benzene, degradation products, and/or some "heavies" (i.e., polychlorinated cyclohexanes, polychlorinated cyclohexenes, and/or polychlorinated cyclohexadienes) are also produced as by-products. In many instances once the desired amount of the desired chlorinated benzene (usually, but not necessarily close to the maximum yield attainable) has been produced, the chlorination is terminated in order to conserve chlorinating agent and to maintain at tolerably low levels the production of undesirable higher chlorinated benzenes and other by-products.
In most instances the formation of by-products is undesirable since the by-products are of lesser value economically and since they often result in disposal problems.